A fluorinated polymer having sulfonic acid type functional groups is widely used for diaphragms (e.g. for alkaline electrolysis, for electrodialysis, for various organic electrolytic syntheses, etc.), and the fluorinated polymer having sulfonic acid type functional groups, is usually a fluorinated polymer having structural units derived from tetrafluoroethylene and structural units having sulfonic acid type functional groups, wherein the proportion of structural units derived from tetrafluoroethylene is approximately from 85 to 90 mol % in all structural units (100 mol %) in the fluorinated polymer (hereinafter referred to also as a fluorinated polymer (X)).
The fluorinated polymer (X) is obtainable in the form of particles of the fluorinated polymer (X), by obtaining solution or dispersion of the fluorinated polymer (X) by means of a solution polymerization method, and then aggregating the fluorinated polymer from the solution or dispersion of the fluorinated polymer (X).
However, if fine particles are formed in a large amount during aggregation of the fluorinated polymer (X) from the solution or dispersion of the fluorinated polymer (X), it would be difficult to separate particles of the fluorinated polymer (X) by filtration and to recover an unreacted monomer.
As a method for producing fluorinated polymer particles while sufficiently suppressing formation of fine particles, the following method has been proposed.
A method wherein, a fluorinated polymer is dissolved or dispersed in a specific good solvent whereby the degree of swelling of the fluorinated polymer is within a specific range, thereby to prepare solution or dispersion of the polymer, then the solution or dispersion of the fluorinated polymer and a specific poor solvent whereby the degree of swelling of the fluorinated polymer is within a specific range, are mixed in such a proportion that the degree of swelling of the fluorinated polymer would be within a specific range by a mixed solvent of the good solvent and the poor solvent, whereby the fluorinated polymer is aggregated to form fluorinated polymer particles (Patent Document 1).
Meanwhile, recently, (1) high cell output (high ion exchange capacity) and (2) high heat resistance (high softening temperature) are required for a fluorinated polymer having sulfonic acid type functional groups to be used for polymer electrolyte membranes, catalyst layers, etc. of fuel cells.
For (1), there is a tendency to reduce the proportion of structural units derived from tetrafluoroethylene and to increase structural units having sulfonic acid type functional groups, in the fluorinated polymer.
For (2), it has been proposed to reduce the proportion of structural units derived from tetrafluoroethylene and to introduce structural units having a cyclic structure and fluorine atoms anew, in the fluorinated polymer (Patent Document 2).
However, in the case of a fluorinated polymer wherein the proportion of structural units having sulfonic acid type functional groups or the proportion of structural units having a cyclic structure and fluorine atoms is increased, while reducing the proportion of the structural units derived from tetrafluoroethylene (hereinafter referred to also as a fluorinated polymer (A)), the solubility of the fluorinated polymer (A) in a good solvent tends to be high. Therefore, the fluorinated polymer (A) will be dissolved in a good solvent without being swollen by the good solvent, i.e. it will not be possible to obtain the degree of swelling of the fluorinated polymer (A) by the good solvent. If it is not possible to obtain the degree of swelling of the fluorinated polymer (A) by a good solvent, it is not possible to apply the production method as described in Patent Document 1 to such fluorinated polymer (A).
Further, at the time of aggregating the fluorinated polymer (A) from the solution or dispersion of the fluorinated polymer (A), fine particles are likely to be formed in a large amount, or particles of the fluorinated polymer (A) are likely to adhere to one another to form large aggregates.